Published Papers

X-Ray Studies of Nucleic Acids. 1947.

Page 03 [68]

< Previous Page     Page 02 [67]

Back to Document Index X-Ray Studies of Nucleic Acids

Next Page > Page 04 [69]

Title:	X-Ray Studies of Nucleic Acids [3 of 13]
Creator:	Astbury, William
Contributor:	Symposia of the Society for Experimental Biology, No. 1
Publisher:
Date:	1947-00-00
Subject:	Nucleic acids -- Structure X-rays -- Diffraction
Description:	Symposia of the Society for Experimental Biology, No. 1
Type:	Text
Format:	text/plain
Language:	en
Identifier:	astbury-pg03
Source:	Master scanned with Epson GT-10000+ flatbed scanner at 600 dpi.
Rights:	http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/copyright.html
Full Text:	68 X-RAY STUDIES Or NUCLEIC ACIDS there are long portions of chains (or columns in this case) of sufficient regularity to build up crystallographically more perfect regions, while at the same time there occurs every so often some chemical peculiarity that might indeed confer specificity on a structure that is otherwise designed to perform some more standard function. These things are matters for future research, and for the moment we must content ourselves with the statement that present X-ray evidence indicates that probably the bulk of a Na thymonucleate fibre is constructed from molecules built to a regular pattern, geometrical or chemical or both, based on a sequence of nucleotides that is a multiple of four. As emphasized in previous communications (Astbury & Bell, 1938), the outstanding feature of the Na thymonucleate diffraction pattern is the strong meridian arc of spacing 3-34 A. that we have associated with a succession of not very dissimilar flat nucleotides lying transversely all along the molecular column. «'e have found no sufficient reason to modify this view, and taking everything into account it still seems by far the most satisfactory inter- pretation. It is not yet certain, however, till more highly oriented pre- parations have been attained, whether the arc lies truly on the meridian, whether it consists of two overlapping layer-line reflexions, or whether-as seems most likely-it combines both these manifestations. Crystallo- graphically speaking, the point is of no great importance at the present stage of trying to deduce the main outlines of the structure: it only means that it is as yet uncertain whether the average thickness of a nucleotide is 3'3,1 A. or a little bit more, say 3-4 A. It might be asked on what grounds has it been inferred that the nucleotides lie immediately on top of one another like a great pile of plates and are not disposed spiralwise round the long axis of the molecule. Arrangements of the latter kind are certainly- not ruled out in general and in reactions with other molecules such as the proteins, but we are dealing here with the equilibrium arrangement, or state of crystallization, of solid Na thymo- nucleate, and the overriding consideration, it must be repeated, is the high density. If successive nucleotides do not lie closely on top of one another, then the state of packing must be dimensionally equivalent to such an arrangement, that is to say, nucleotides of neighbouring molecules must be closely interleaved in a surprisingly regular fashion. This seems highly unlikely, for the stabilizing forces of the structure will be chiefly the strong total attractions between the large flat areas, and the simplest and most effective way of reducing the potential energy to a minimum is for such areas to lie, if possible, directly on top of one another, as happens in other structures built from flat units. The stiffness and the estimated thickness (see Greenstein, 1944, p. zoo, table) of rod-like units that have been